1. FIELD OF THE INVENTION:
The present invention relates to a semiconductor memory device and, more particularly, to a EPROM, which is an "electrically programmable read only memory."
2. DESCRIPTION OF THE PRIOR ART:
A user of the EPROM can write information (such as a program or digital data) into the EPROM. The written information is often checked or verified to ensure its correctness. This checking or verification is performed by a specific operation which is called a "verification operation". For example, binary information is first written into a memory cell by applying a high program voltage to a program terminal of the EPROM. Immediately thereafter, the information is read out, through an output terminal of the EPROM. The read-out information is verified by being compared with the correct information. If the read-out information is correct, the correct information is again written into the cell.
It should be noted that the verification operation is performed at a timing when the information is written into the memory and immediately after the information is written into the memory cell. The programming voltage, which is higher than a normal driving voltage, is applied through a digit line to an addressed memory cell. The written information is also read out of the cell through the digit line and a sense amplifier immediately after the writing operation is completed to the addressed cell.
As described above, the digit line is used for both the writing operation and the verification operation. Since the high programming voltage (e.g., 20V) is applied to the digit line in the writing operation, the digit line is charged to a high potential, in the writing operation. Since this charge remains on the digit line after the writing operation has been completed, the digit line must be discharged before the read out operation. If not, the read-out information would be changed by the charges remaining on the digit line, and no accurate verification would be possible.
In the prior art, the remaining charge (residual charge) is discharged through the memory transistors, before the verification operation. Hence, a long period of time is required between the writing (programming) operation and the verification operation. Further, the memory transistors into which the information has been written have a threshold level which is shifted to a high level, so that the impedance of the memory transistor becomes high. Therefore, the discharge period becomes longer.
In addition, a high voltage is applied to a transistor or transistors connected to the digit line (e.g., a digit line selecting transistor, a program control transistor, or a memory transistor) during the discharge period, because of the high impedance memory transistor. Therefore, there are shortcomings in the prior art because the transistor or transistors connected to the digit line are destroyed by the high voltage. To avoid this destruction, a transistor with a high withstand voltage or a high breakdown voltage is required. However, since such a transistor occupies a large area on a semiconductor chip, it is difficult to form a large capacity memory circuit on the chip.
An object of the present invention is to provide a semiconductor memory device in which a digit line charged by a programming (writing) operation can be discharged at a high speed.
Another object of the present invention is to provide a programmable memory device which can perform a verification operation at a high speed.
Still another object of the present invention is to provide, on a semiconductor chip, a programmable memory device having a large memory capacity and having a smooth verification function.